1. Field of the Invention
This invention relates to an ultra-violet (UV) absorbence-based monitor for measuring the amount of organic pollution in a liquid, and is of particular application for on-line monitoring of water quality at locations, such as, for example, industrial or water treatment plant effluent outlets, rivers or reservoirs.
2. Related Art
A number of methods are available for determining the biological or organic pollution in water, involving the measurement of parameters such as BOD (biochemical oxygen demand), TOC (total organic carbon) or COD (chemical oxygen demand). The parameter that is usually of most interest is the BOD. Traditionally, the SOD of a liquid sample has been determined in a laboratory test in which a liquid sample is incubated in the presence of microorganisms for 5 days at 20.degree. C. in the dark, dissolved oxygen consumed by the micro-organisms being measured and expressed as BOD in mg O.sub.2 /L of sample. The 5 day BOD test is a standard test laid down by a Royal Commission in 1910. It gives results that are repeatable either for different samples of the same liquid, or for the same liquid tested at different laboratories, within about 15 per cent, but it is, however, exceedingly cumbersome and slow; hence, a need has arisen for a method of continuously monitoring BOD on-line at influent or effluent locations both to meet legislative requirements and to provide process control information.
A number of on-line, continuous BOD monitors have recently been developed, based on a variety of techniques. One such monitor is an on-line continuous BOD monitor based on the respiratory method. Although that monitor is capable of producing similar results to the laboratory test over a three minute rather than five day period, it is prohibitively expensive.
Alternative monitors include the cheaper UV absorbence-based monitors, which are simple to use and maintain. These are based on the principle that a particular substance will absorb light of a particular wavelength (or wavelengths), so that the reduction in intensity of light of that wavelength, when transmitted through a sample containing the substance, can be related to the concentration of that substance. Aromatic organic compounds, and other organic compounds that have conjugated double bonds, absorb light in the UV wavelength region and research has shown that absorbence measurements taken in that region, and in particular at 254 nm, can be related to BOD (and also to TOC and COD).
The performance of existing UV based-absorbence monitors has, however, been disappointing in that the absorption readings (usually at 254 nm) have been shown to be unreliable as an indication of organic pollution due to their dependence on other, unknown factors. Past attempts have been made to compensate only for turbidity in liquid samples. While these monitors have proved accurate and reliable, there being a good correlation between their results and the standard 5-day BOD test, this has only been because the use of such monitors has been confined to specific locations, liquid types and to short sampling periods (less than one day), with thorough testing first being carried out in the laboratory in order to determine the correlation between the UV absorbence and the measured parameter.